102 MAZATLAN BIVALVES 
centric ridges slightly decussated. They are evidently young, 
extremely thin, and shaped like the fossil F. lamellosa. 
Leng. 06, lat. ‘08, alt. “04. 
Hab.—Mazatlan ; off Spondylus caleifer, 2sp.; Z’pool Col. 
Genus DIPLODONTA, Bronz. 
Diplodonta, Bronn 1831, Italiens Tertiargeb. p. 9. 
Mysia, Gray, Proc. Zool. Soc. 1847, p. 195, (non Mysia Leach,) 
teste Phil. in Handb. der Conch. & Mal. p. 342. 
Glocomene, Leach (a secundé manu,) Br. Moll. 318. 
For a full account of the remarkable peculiarities of the 
animal of D. rotundata, v. Clark Moll. Test. Mar. Br. p. 82. 
150. DtetoponTs sEMIASPERA, ? Phil. 
Quoted by Philippi, in Abbild. Conch. Tellina, p. 25, under 
Tellina pisiformis, Aug. 1846. 
?=ITmcina orbella; Gould, Proc. Bost. Soc. N. H. Nov. 1851, 
vol. iv. p. 90:—Cal. & Mex. Shells, p. 22, pl. 15, f. 3. 
?=Lucina semireticulata, B. WM. Cat. D’Orb. Moll. p. 72, 
no. 640 :—Do. Cuba Moll. p. 41, no. 498. 
_ P=Lucina celata, Rve. Conch. Ic. pl. 6, f. 27, a, 5. 
A small colony was found ina burrow in Chama, old and 
young, most tightly wedged. They apparently lived on till 
they smothered each other. The shape of individuals varies 
extremely. Some are nearly globular, others (smaller) much 
flattened. Some are solid, some with a glossy sheli like 
Kellia. In some the ligament was conspicuous outside; in 
others entirely hidden. The outside is sometimes nearly 
smooth ; sometimes with concentric lines of growth, here and 
there granulose. The ligamental plate is generally more or 
less turned in; the teeth rather divergent and strongly bifid. 
The muscular scars are rather long, especially the posterior 
one, serrated within, and (with the pallial line) near the mar- 
gin; but even these characters seem to vary in every specimen. 
The umbo in the very young shell is subcentral, with the hga- 
ment entirely external, and the hinge teeth prominent. The 
smallest specimen measures ‘O04 across; the largest, long. °41, 
lat. °42, alt. °28. 
After a very careful comparison of the types of L. celata, L. 
orbella and D. semiaspera, I am unable to detect differences 
between them which do not exist between specimens of the 
